A diphthong is best defined as:

A diphthong is defined as a complex vowel sound that glides from one vowel sound to another within a syllable. This characteristic is what distinguishes diphthongs from simple vowels. When producing a diphthong, the speaker transitions smoothly between two vowel qualities, which creates a unique, dynamic sound that is quite different from a single, static vowel sound.

For instance, in the English word "coin," the vowel sound begins at an open position and glides to a closer position, thereby forming a diphthong. This fluid movement is essential to understanding how diphthongs function in phonetics.

Other options suggest misunderstandings of what constitutes a diphthong. A single vowel sound does not involve this gliding movement, consonant sounds are distinctly different phonetic elements unrelated to vowel behavior, and a combination of a vowel and a consonant sound does not accurately capture the essence of a diphthong's characteristics.